The field of the invention relates to control systems for maintaining engine air/fuel operation within the peak efficiency window of a catalytic converter.
Several closed-loop air/fuel control systems are known which utilize dual exhaust gas oxygen (EGO) sensors, one upstream and one downstream of a catalytic converter. Ideally, the output of the upstream EGO sensor should have a step change at stoichiometry. It has been found, however, that the sensor step change tends to shift from stoichiometry as a result of component aging and other sensor properties such as response time asymmetry. Overall system characteristics such as incomplete exhaust gas mixing may also cause shifts away from peak catalyst efficiency. Moreover, the efficiency window of the catalyst may not be at stoichiometry. Accordingly, there may be a mismatch between the sensor step change output and the catalyst efficiency window.
Several methods of biasing have conventionally been used to address this sensor-catalyst mismatch. For example, the sensor output is typically compared to a reference value at the midpoint of its step to generate a rich or lean indication, and the reference may be changed from the midpoint to bias the rich/lean indication. However, this biasing method is limited to a very narrow range of air/fuel values because of the loss of sensor sensitivity away from the narrow linear region around the midpoint. For those systems which employ integral or proportional plus integral feedback control, biasing may be added to the feedback controller. For example, the integration rate in the lean direction may be changed from the rich direction. However, the resulting asymmetry may result in periodic engine operation outside of the converter's efficiency window.